There are numerous applications calling for the joining of elongated elements juxtaposed along a coextensive joint or seam. For most metallic elements, the obvious approach is to join them with a seam weld. There are of course numerous types of welding instrumentalities that may be utilized to effect seam welds on a production line basis. Such as TlG, MlG, electron beam, laser welding, etc. Of these, laser welding offets the advantage of being capable of concentrating the Welding energy to extremely high intensities onto very small areas with great precision.
As a result, laser welds can be effected with substantially less heat input to the elements as compared to TlG or MlG welds.. Residual tensile stresses are dramatically reduced, as are adverse metallurgical changes in the elements in the heat affected zones of the elements adjacent the weldment.
Whichever welding technique is utilized, seam welding an elongated joint on an automated production line basis requires a tracking sensor to locate and follow the joint and then to positional control the application of welding energy during relative motion of the joint and the welding instrumentality. In the case of laser welding, the tracking sensor must continuously resolve the joint location with high precision, as the laser beam impinging on the joint may be focused down to a diameter as small as 10 mils or less. Tracking sensors may be of the contacting or non-contacting type. Those using a contacting stylus are capable of following a joint with a reasonable degree of resolution to control the application of a laser welding beam thereto. However, the stylus tends to wear over time, and reliability suffers as a result. Non-contacting sensors utilizing electrical, optical, pneumatic or acoustic probing techniques are potential candidates.
A particularly attractive non-contacting tracking sensor approach is the one disclosed in commonly assigned Penney et al., U.S. Pat. No. 4,645,917, entitled "Swept Aperture Flying Spot Scanner". As disclosed therein, a laser beam, transmitted through a coherent optic fiber bundle, is projected at a suitable angle, e.g. 30.degree., to normal, to a focused spot substantially at the surface to be profiled. The laser spot is swept over the surface in X and Y raster scan fashion. The diffuse, reflected light from the scanned laser spot observed along a line of sight normal to the surface is conveyed through a second coherent fiber optic bundle to a descanning device, such as a mirror oscillating in synchronism with the movement of the laser spot in one of the sweep directions, e.g., Y direction. This produces a narrow scanning aperture through which the diffuse, reflected light passes to a suitable optical sensor, such as a photomultiplier tube. By noting at what point in the interval of each X-direction sweep the laser spot is observed by the optical sensor, the elevation of the observed surface as a function of where the laser spot is in the Y-direction sweep can be determined by optical triangulation. From this can be obtained a precise profile of the surface in the field of view defined by the scanning aperture, which lies in a viewing plane normal to the surface. That is, the optical sensor response can be processed to provide, in essence, a sectional view indicating surface profile. An application calling for effecting a plurality of precise, elongated seam welds, to which the present invention has particular applicability, is the manufacture of control rods for nuclear reactors. In the commonly assigned, copending application of Aiello et al., Ser. No. 250,631, filed Sept. 29, 1988, entitled "Control Rod Absorber Section Fabrication by Square Tube Configuration and Dual Laser Welding Process", there is disclosed apparatus for manufacturing control rods of cruciform cross section. Each control rod is comprised of a multiplicity of elongated tubes, each having a basically square exterior shape and a central bore for accepting neutron absorbing or so-called "poison" material, such as boron carbide. The individual tubes, with the poison sealed therein, are clamped together in a fixture with as many as fourteen tubes in each of the four wings of the cruciform. The corners of the tubes are chamfered, such that when juxtaposed along their lengths of fourteen feet more or less, a multiplicity of parallel V-shaped grooves or seams between adjacent tubes are presented. The fixtured tubes are conveyed through a welding station where a cruciform wing passes between a pair of laser welders which are controlled to effect a continuous seam weld in a V-shaped groove on each side of the wing. After multiple passes through the welding station, all of the tubes are united by welded seam joints. The cruciform wings are welded to central tie bar sections, a handle is welded to one end of the cruciform, and a velocity limiter is welded to the other end to create a structurally rigid control rod capable of withstanding all foreseeable static and dynamic loadings.
The individual tubes though long in length, are rather small in cross-section, e.g., measuring as little as 0.260 inches on each side. Thus, the V-shaped groove between adjacent tubes is extremely small, with an opening width of 30 mils or less. To resolve a joint of such minute lateral dimensions and to faithfully track it while moving at speeds in excess of 60 inches per minute is a formidable task. Moreover, the tracking sensor must control the laser welding beam such that it is both laterally aligned with the groove and focused substantially at the bottom thereof all the while the fixtured tubes are being conveyed through the welding station at a high relative velocity.
It is accordingly an object of the present invention to provide a laser welding system for joining metallic elements along an elongated joint.
A further object is to provide a laser welding system of the above-character, wherein the laser welding beam is controlled to automatically track an elongated joint while moving at a relatively high velocity.
An additional object is to provide a joint tracking laser welding system of the above-character, which is capable of precision welding an elongated joint having relatively small transverse dimensions.
Another object is to provide a precision joint tracking laser welding system of the above-character, which is flexible in operation, insensitive to environmental influences, and convenient to implement.
Other objects of the invention will in part be obvious and in part appear hereinafter.